These efforts cover both basic and applied aspects of mycoplasmas and related wall-free prokaryotes, including their neurotoxins, antigens, and other biological factors involved in virulence, their immunological interrelationships, and their possible role in human disease or diseases of uncertain etiology. Current projects concern the characterization, pathogenicity, and serological properties of an expanding group of helical mycoplasmas (spiroplasmas) being isolated from plants and a variety of insects (especially ticks). Seven new spiroplasma strains have been isolated from Ixodes pacificus ticks collected in Oregon. These organisms appear to represent a completely new, and serologically-distinct, group of spiroplasmas. Serological analysis of most known spiroplasmas has shown at least six distinct serogroups, as based upon the spiroplasma metabolism inhibition test developed in this laboratory. This information will assist sero-epidemiological studies on human sera with these organisms. Experimental infections of suckling rats with a pathogenic spiroplasma has failed to show a clear inflammatory response in brain tissue, despite the fact that the animals have central nervous system symptoms and as many as 10 to the 5th power infectious units/gram of brain tissue. However, spongiform changes were seen.